fdt_support.c: Correct linux,initrd-start/end setting

The change to add 64bit initrd support broke 32bit initrd support as it
always set 64bits worth of data into the properties, even on 32bit
systems. The fix is to use addr_cell_len (which already says how much
data is in 'tmp') to set the property, rather than always setting 8.
Thanks to Stephen Warren for pointing out the fix here.
Reported-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Tom Rini <trini@ti.com>